mfbt/PodOperations.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 /*
   8  * Operations for zeroing POD types, arrays, and so on.
   9  *
  10  * These operations are preferable to memset, memcmp, and the like because they
  11  * don't require remembering to multiply by sizeof(T), array lengths, and so on
  12  * everywhere.
  13  */
  14
  15 #ifndef mozilla_PodOperations_h
  16 #define mozilla_PodOperations_h
  17
  18 #include "mozilla/Array.h"
  19 #include "mozilla/ArrayUtils.h"
  20 #include "mozilla/Attributes.h"
  21
  22 #include <algorithm>
  23 #include <stdint.h>
  24 #include <string.h>
  25
  26 namespace mozilla {
  27
  28 /** Set the contents of |aT| to 0. */
  29 template <typename T>
  30 static MOZ_ALWAYS_INLINE void PodZero(T* aT) {
  31   memset(aT, 0, sizeof(T));
  32 }
  33
  34 /** Set the contents of |aNElem| elements starting at |aT| to 0. */
  35 template <typename T>
  36 static MOZ_ALWAYS_INLINE void PodZero(T* aT, size_t aNElem) {
  37   /*
  38    * This function is often called with 'aNElem' small; we use an inline loop
  39    * instead of calling 'memset' with a non-constant length.  The compiler
  40    * should inline the memset call with constant size, though.
  41    */
  42   for (T* end = aT + aNElem; aT < end; aT++) {
  43     memset(aT, 0, sizeof(T));
  44   }
  45 }
  46
  47 /*
  48  * Arrays implicitly convert to pointers to their first element, which is
  49  * dangerous when combined with the above PodZero definitions.  Adding an
  50  * overload for arrays is ambiguous, so we need another identifier.  The
  51  * ambiguous overload is left to catch mistaken uses of PodZero; if you get a
  52  * compile error involving PodZero and array types, use PodArrayZero instead.
  53  */
  54 template <typename T, size_t N>
  55 static void PodZero(T (&aT)[N]) = delete;
  56 template <typename T, size_t N>
  57 static void PodZero(T (&aT)[N], size_t aNElem) = delete;
  58
  59 /** Set the contents of the array |aT| to zero. */
  60 template <class T, size_t N>
  61 static MOZ_ALWAYS_INLINE void PodArrayZero(T (&aT)[N]) {
  62   memset(aT, 0, N * sizeof(T));
  63 }
  64
  65 template <typename T, size_t N>
  66 static MOZ_ALWAYS_INLINE void PodArrayZero(Array<T, N>& aArr) {
  67   memset(&aArr[0], 0, N * sizeof(T));
  68 }
  69
  70 /**
  71  * Assign |*aSrc| to |*aDst|.  The locations must not be the same and must not
  72  * overlap.
  73  */
  74 template <typename T>
  75 static MOZ_ALWAYS_INLINE void PodAssign(T* aDst, const T* aSrc) {
  76   MOZ_ASSERT(aDst + 1 <= aSrc || aSrc + 1 <= aDst,
  77              "destination and source must not overlap");
  78   memcpy(reinterpret_cast<char*>(aDst), reinterpret_cast<const char*>(aSrc),
  79          sizeof(T));
  80 }
  81
  82 /**
  83  * Copy |aNElem| T elements from |aSrc| to |aDst|.  The two memory ranges must
  84  * not overlap!
  85  */
  86 template <typename T>
  87 static MOZ_ALWAYS_INLINE void PodCopy(T* aDst, const T* aSrc, size_t aNElem) {
  88   MOZ_ASSERT(aDst + aNElem <= aSrc || aSrc + aNElem <= aDst,
  89              "destination and source must not overlap");
  90   if (aNElem < 128) {
  91     /*
  92      * Avoid using operator= in this loop, as it may have been
  93      * intentionally deleted by the POD type.
  94      */
  95     for (const T* srcend = aSrc + aNElem; aSrc < srcend; aSrc++, aDst++) {
  96       PodAssign(aDst, aSrc);
  97     }
  98   } else {
  99     memcpy(aDst, aSrc, aNElem * sizeof(T));
 100   }
 101 }
 102
 103 template <typename T>
 104 static MOZ_ALWAYS_INLINE void PodCopy(volatile T* aDst, const volatile T* aSrc,
 105                                       size_t aNElem) {
 106   MOZ_ASSERT(aDst + aNElem <= aSrc || aSrc + aNElem <= aDst,
 107              "destination and source must not overlap");
 108
 109   /*
 110    * Volatile |aDst| requires extra work, because it's undefined behavior to
 111    * modify volatile objects using the mem* functions.  Just write out the
 112    * loops manually, using operator= rather than memcpy for the same reason,
 113    * and let the compiler optimize to the extent it can.
 114    */
 115   for (const volatile T* srcend = aSrc + aNElem; aSrc < srcend;
 116        aSrc++, aDst++) {
 117     *aDst = *aSrc;
 118   }
 119 }
 120
 121 /*
 122  * Copy the contents of the array |aSrc| into the array |aDst|, both of size N.
 123  * The arrays must not overlap!
 124  */
 125 template <class T, size_t N>
 126 static MOZ_ALWAYS_INLINE void PodArrayCopy(T (&aDst)[N], const T (&aSrc)[N]) {
 127   PodCopy(aDst, aSrc, N);
 128 }
 129
 130 /**
 131  * Copy the memory for |aNElem| T elements from |aSrc| to |aDst|.  If the two
 132  * memory ranges overlap, then the effect is as if the |aNElem| elements are
 133  * first copied from |aSrc| to a temporary array, and then from the temporary
 134  * array to |aDst|.
 135  */
 136 template <typename T>
 137 static MOZ_ALWAYS_INLINE void PodMove(T* aDst, const T* aSrc, size_t aNElem) {
 138   MOZ_ASSERT(aNElem <= SIZE_MAX / sizeof(T),
 139              "trying to move an impossible number of elements");
 140   memmove(aDst, aSrc, aNElem * sizeof(T));
 141 }
 142
 143 /**
 144  * Looking for a PodEqual? Use ArrayEqual from ArrayUtils.h.
 145  * Note that we *cannot* use memcmp for this, due to padding bytes, etc..
 146  */
 147
 148 }  // namespace mozilla
 149
 150 #endif /* mozilla_PodOperations_h */